Link: THE EMERGING THREE-TIER PRICING MODEL FOR PRIVATE CLOUD

Private clouds owned and self-managed by enterprises can be cheaper than public cloud. The magic number to beat is about $25 per VM-month at 100% utilization. If the cost of the whole stack comes in under this number, then even with the addition of labor to manage that private cloud, it should be cheaper than public cloud. Obviously, with better labor efficiency, unit costs versus public cloud are lowered further, and the relative value of benefits increases. Enterprises unable to achieve a labor efficiency of 300 VMs per engineer are unlikely to beat public cloud on price.
Original source: THE EMERGING THREE-TIER PRICING MODEL FOR PRIVATE CLOUD